This section provides background information related to the present disclosure which is not necessarily prior art.
Free radical photopolymerization is a common materials fabrication technique used in coatings, microfabrication, and 3D printing, in particular stereolithography. Under ambient conditions molecular oxygen is present in the photopolymerizing material, called the photoresin. When light hits the photoresin, free radicals induce polymerization and solidification. Typically molecular oxygen exists in a triplet quantum state under normal conditions, and reacts with the free radicals to terminate the polymerization reaction, known as oxygen inhibition.
Oxygen inhibition can outright prevent certain liquid photoresins from polymerizing, a major problem for coatings performed under ambient conditions. Oxygen inhibition also has major effects on 3D printing processes, like stereolithography, such as limiting network density and feature accuracy. However, oxygen inhibition can also be useful to stop the reaction if controlled properly.
Several methods have been developed in order to control oxygen inhibition; the majority being summarized by Ligon et al. (“Strategies to Reduce Oxygen Inhibition in Photoinduced Polymerization”, Chem. Rev. 2014, 114, pp. 557-589). Methods typically include performing polymerization under inert, nitrogen, atmospheres which requires pressurized gasses and suitable plumbing. Another method is incorporation of photosensitive chemicals, called photosensitizers. Photosensitizers when irradiated with light, of a wavelength unique to the photosensitizer, react with triplet oxygen to form singlet oxygen. Triplet and singlet oxygen have different chemistries, with singlet oxygen being more reactive than triplet oxygen. This increase in reactivity allows singlet oxygen to react with substrates typically unaffected by triplet oxygen giving it several uses including in photodynamic therapy, sterilization, and fine chemical synthesis summarized by DeRosa et al. (“Photosensitized singlet oxygen and its applications”, Coord. Chem. Rev. 2002, 233-234, pp. 351-371). Certain chemicals are used, called quenchers, which do not react with triplet oxygen, but do react with singlet oxygen, and can be used to remove oxygen from the photoresin. The quenched oxygen can no longer participate in oxygen inhibition of radical polymerization, allowing the reaction to proceed uninhibited until new oxygen diffuses in and inhibits the reaction. The light used to stimulate the photosensitizes, create singlet oxygen, and prevent oxygen inhibition is uncoupled from the polymerization which typically occurs at a different wavelength. Scranton et al. (“Photochemical Method to Eliminate Oxygen Inhibition of Free Radical Polymerizations,” Nov. 28, 2006, U.S. Pat. No. 7,141,615 B2) describes the use of photosensitizers to control oxygen inhibition during photo-polymerization.
All of the above discussed techniques used to control oxygen inhibition have a common drawback in that they rely on photosensitizers to excite triplet oxygen to single oxygen, and they lack spatial-temporal control.
Lithography and additive manufacturing are continually driving toward greater control over features size and being able to manufacture these features over large areas. Spatial temporal control of where oxygen inhibition occurs would be highly valuable in order to gain an even greater degree of control over the size of features of additively manufactured parts. This is especially important because as the manufacturing footprint of the part grows, it becomes increasingly difficult and expensive to control oxygen inhibition by inert atmospheres as has been done traditionally. Oxygen can be thought of as a naturally occurring ambient polymerization inhibitor, and previous work has shown that spatial temporal control of inhibitor can lead to resolutions beyond the abbe diffraction limit summarized in the work of Scott et al. (“Two-Color Single-Photon Photoinitiation and Photoinhibition for Subdiffraction Photolithography”, Science, 2009, 324 (5929), pp. 913-917).
The use of a photosensitizer is limiting due to cost, material compatibility with increasingly diverse photoresin systems, and the kinetic complexities of multi-component chemical reactions with photosensitizer reacting with triplet oxygen to form singlet oxygen, and then singlet oxygen reacting with quencher. Simplification of this system would be highly beneficial to remediate these deficiencies. Direct excitation is one way to simplify this process by eliminating the need of a photosensitizer. In this method, light of a specific wavelength, for example typically 765 nm, 1064 nm, or 1273 nm wavelength light, will cause the photoexcitation of triplet oxygen to the singlet state. Light having a wavelength of 765 nm is more favorable due to widely available light sources and avoidance of being within the absorbance band of water. And for the sake of brevity, the present disclosure will refer to 765 nm as the wavelength of choice, but it will be understood that this wavelength of light is but one of several different wavelengths that is able to cause the desired photoexcitation of triplet oxygen to the singlet state, and therefore the present disclosure is not limited to use with only one specific wavelength of light. Additional background and benefits of this simplification applied specifically to photodynamic therapy can be found in the work of Bregnhøj et al. and citations therein (“Direct 765 nm optical excitation of molecular oxygen in solution and in single mammalian cells”, J. Phys. Chem. B, 2015, 119 (17), pp. 5422-5429).
The benefits of direct excitation and spatial temporal control of oxygen has yet to be introduced to the manufacturing realm. Unpatterned light can be used to directly excite ambient oxygen within a photoresin to singlet oxygen which can then be subsequently removed by a quencher allowing thin, large area photoresins previously needing to be cured under inert atmosphere to be done under ambient conditions. There are additional benefits for additive manufacturing where both the simplified chemical process and spatial temporal control of light can be leveraged.